1. Field of the Invention
The present invention relates to a method and apparatus for passively compensating for thermal expansion and, more particularly, for precisely controlling the spacing between two objects such as lenses throughout a temperature range.
2. Discussion of Related Art
Physical objects undergo changes in dimensions and other characteristics in response to a change in ambient temperature. In some applications, this change can be inconsequential, while with others the change can be quite significant. Dimensional expansion or contraction of a support structure can affect the spacing between two objects in an undesirable manner. For instance, it is well known that the imaging performance of an optical system is dependent on temperature. Increases and decreases in ambient temperature change the physical dimensions of both the lens elements as well as the mechanical components of an optical system. Also, the refractive properties of the lens elements change with changes in ambient temperature.
To address this problem, many solutions have been proposed. For instance, U.S. Pat. Nos. 1,325,936 to Fouasse and 2,533,478 to Lee et al. disclose temperature compensating means for lens mounts which use two materials having different coefficients of thermal expansion. In the Fouasse patent, each lens of a lens system is mounted to a respective tube, one tube being inside the other. Each tube is made of a different material having a different coefficient of thermal expansion such that when the lens system undergoes a temperature change, each tube expands at a different rate. These different rates control the spacing of the two lens elements relative to each other. In these systems, however, the thermal expansion or contraction must be in the direction of the optical axis, which requires relatively large spans of material along the optical axis, making the system bulky and relatively difficult and expensive to manufacture. Furthermore, the lenses must be mounted in a cantilevered fashion, i.e., spaced from the support structure by the respective mounting tubes. This leads to vibration harmonics and other undesirable mechanical affects which tend to deteriorate lens system performance.
The Lee et al. patent discloses selective control of the spacing of a lens group from a surface to maintain the focal point of the lens group at a predetermined position through a temperature change. Again, a cantilevered mounting structure incorporating materials of different coefficients of thermal expansion are used to control the relative spacing of the lens group from its focal plane.
Other systems, such as disclosed in U.S. Pat. Nos. 5,313,333 and 5,210,650 both to O'Brian et al., use a lever action to enhance the amount of movement of the lens relative to the amount of thermal expansion of the mounting structure along the optical axis of the system.
A review of previous passive athermalization optical assemblies reveals that the solutions are bulky, difficult to manufacture and subject to mechanical vibration and other stresses and generally not conducive to practical applications.
Other systems include intervention of positioning means such as lens drive motors and rotational lens mounts wherein focus is maintained through some form of feedback controlling the lens drive, which in turn controls the relative positioning of the lens group to maintain focus. This type of thermal compensation adds greatly to the complexity and weight of the optical system.